Phosphatases remove phosphate groups from molecules previously activated by kinases and control most cellular signaling events that regulate cell growth and differentiation, cell-to-cell contacts, the cell cycle and oncogenesis. Protein phosphorylation is the ubiquitous strategy used to control the activities of eukaryotic cells. It is estimated that more than 1000 of the 10,000 proteins active in a typical mammalian cell are phosphorylated. The high energy phosphate which confers activation is transferred from adenosine triphosphate molecules to a protein by protein kinases, and is subsequently removed from the protein by protein phosphatases.
There appear to be three, evolutionarily-distinct protein phosphatase gene families (Carbonneau H and Tonks NK (1992, Annu Rev Cell Biol 8:463-93). They are the protein phosphatases (PP), the protein tyrosine phosphatases (PTP) and the acid/alkaline phosphatases (AP). Although APs dephosphoryate substrates in vitro, their role in vivo is not well known.
PTPs remove phosphate groups only from selected phosphotyrosines on particular types of proteins. In so doing, PTPs reverse the effects of protein tyrosine kinases (PTK) and therefore play a significant role in cell cycle and cell signaling processes. PTPs possess a high specific enzyme activity relative to their PTK counterparts and therefore ensure that tyrosine phosphorylations are very short lived and very uncommon in resting cells. Many PTKs are encoded by oncogenes, and it is well known that oncogenesis is often accompanied by increased tyrosine phosphorylation activity. It is therefore possible that PTPs may serve to prevent or reverse cell transformation and the growth of various cancers by controlling the levels of tyrosine phosphorylation in cells. This is supported by studies showing that overexpression of PTP can suppress transformation in cells and, conversely, specific inhibition of PTP can enhance cell transformation.
The PTPs are found in transmembrane, receptor-like and nontransmembrane, non-receptor forms, and possess a diversity in size (20 kDa to greater than 100 kDa) and structure. All PTPs share homology within a region of 240 residues which delineates the catalytic domain and contain a conserved sequence, e.g., residues 99 through 107 of SEQ ID NO:5 or residues 102 through 110 of SEQ ID NO:6, near the carboxy terminus. The combination of the catalytic domain with a wide variety of structural motifs accounts for the diversity and specificity of these enzymes. In the nonreceptor isoforms, the noncatalytic sequences may also confer different modes of regulation and target PTPs to various intracellular compartments.
Receptor-like PTPs (R-PTPs) are generally large (greater than 100 kDa) and are grouped on the basis of their single transmembrane segment and two, tandem PTP domains within the cytoplasmic tail. In contrast to the similarity within the internal cytoplasmic domains of these molecules, there is considerable diversity among the extraceliular segement. Key examples of this type of PTP are CD45, a PTP found on the surface of leucocytes that helps to activate T and B lymphocytes when activated by extracellular antibodies and LAR, a PTP having structural features related to the N-CAM family of cell adhesion molecules on its extracellular domain and which may be involved in cell adhesion processes.
Nonreceptor PTPs (NR-PTP) are generally smaller (about 50 kDa) than the R-PTPs and have single catalytic domains and noncatalytic sequences of variable length positioned at either the N- or C-termini. NR-PTPs are intracellular and may use their noncatalytic sequences to direct them to particular subcellular compartments or to determine their enzyme regulating activity. Some NR-PTPs may be divided into subfamilies based on similarities in their noncatalytic domains. For example, human PTPH1 and MEG01 contain homologous catalytic domains and N-terminal segments with homology to band 4.1, talin and ezrin. In addition they are thought to be localized between actin stress fibers and the plasma membrane where they modulate cytoskeletal dynamics. T-cell PTP and PTP1.beta. display a high degree of similarity in their catalytic domains and structural similarities in their C-terminal noncatalytic domains that may help direct them to membranes where they regulate enzyme activity.
Recently, a new class of smaller (about 20 kDa) NR-PTPs has been found which have a single catalytic domain and are represented by PRL-1, found in regenerating rat liver and hepatoma cells, and OV-1, found in human ovarian tissue (Diamond R H, et al (1994) Mol Cell Biol 14:3752-62; Montagna M et al (1995) Hum Genet 96: 532-538). These PTPs possess homology to other NR-PTPs only within the region of the catalytic active site. Stably transfected cells that overexpress PRL-1 exhibit altered cell growth and morphology and a transformed phenotype. Furthermore, it is postulated that PRL-1 is important in the control of normal cell growth and in the development of tumorigenicity.
It is apparent that PTPs may serve either as positive or negative regulators of cell growth, and that a detailed understanding of phosphatase interaction in signal transduction pathways should reveal many potential mechanisms to provide the means for clinical diagnosis or therapeutic intervention in the progression of cancer, inflammatory illnesses, or oncogenesis. The discovery of new PTPs may satisfy a need in the art by providing agents which are useful for the prevention or treatment of HPTP-1-associated diseases.